Fiber-reinforced resin compositions including a thermoplastic resin and reinforcing fibers are easily molded and processed by virtue of the characteristics of the thermoplastic resin, do not require a load in storage unlike thermosetting resins, and are excellent in recyclability. As these fiber-reinforced resin compositions, those in a variety of forms are known such as thermoplastic prepregs in which reinforcing fibers are arranged in the form of a sheet, and pellets in which reinforcing fibers are randomly dispersed. Fiber-reinforced resin compositions are excellent in balance between lightness and dynamic characteristics, and are therefore widely used as structural members of aircraft, automobiles, watercraft and the like, electronic equipment housings, sporting goods, and industrial materials such as building materials.
Among thermoplastic resins, a polyarylene sulfide is particularly excellent in heat resistance and chemical resistance so that a fiber-reinforced resin composition produced using the resin can be expected to be applied as an alternative to a metal material. However, when a fiber-reinforced resin composition produced using a polyarylene sulfide is developed as an alternative to a metal material, further improvement of the dynamic characteristics, particularly the tensile strength and elongation, of the fiber-reinforced resin composition is desired. This is because, since the tensile elongation of a general polyarylene sulfide is lower than the tensile elongation of reinforcing fibers (e.g. about 2% in the case of carbon fibers), the reinforcing effect of reinforcing fibers cannot be sufficiently utilized in the fiber-reinforced resin composition.
A general method of improving the tensile strength and elongation of a fiber-reinforced resin composition is enhancement of the elongation of a polyarylene sulfide to be used. However, tensile elongation of a polyarylene sulfide correlates to its molecular weight and hence its melt viscosity and, when the tensile elongation of the polyarylene sulfide is improved, the melt viscosity increases so that there is a risk of considerably impairing the molding processability of a fiber-reinforced resin composition. It is well known that it becomes harder to combine a polyarylene sulfide with reinforcing fibers as the melt viscosity of the polyarylene sulfide increases. In that case, it is necessary to make the process temperature higher, and therefore a polyarylene sulfide is not suitable to easily produce a fiber-reinforced resin composition with high productivity. For these reasons, it is an important technical challenge to improve the tensile strength and elongation while securing productivity and molding processability in a fiber-reinforced resin composition produced using a polyarylene sulfide.
Japanese Patent Laid-open Publication No. 2008-231291 discloses a molding material in which a high-molecular-weight thermoplastic resin is disposed in contact with a composite including a polyarylene sulfide prepolymer and continuous reinforcing fibers. In that molding material, a continuous reinforcing fiber bundle is impregnated with a low-molecular-weight material, and a high-molecular-weight material is used for a matrix resin so that dynamic characteristics as well as productivity are secured. The polyarylene sulfide prepolymer is an excellent material easily impregnated in a reinforcing fiber bundle to improve productivity of a molding material and easily dispersed in or made compatible with a matrix resin in a molding step to improve dispersion of reinforcing fibers in a molded article. However, the polyarylene sulfide prepolymer has a low molecular weight, and therefore has the problem that the added amount thereof and the dynamic characteristics of the resulting molded article are in a trade-off relationship.
On the other hand, various substances have been studied as additives to be added in a polyarylene sulfide. Japanese Patent Laid-open Publication No. 10-168290 discloses a composition in which a polyarylene sulfide, a cyclic polyolefin resin and a polycarbodiimide are combined to improve adhesion with a metal material and an organic material. For that composition, however, reinforcing fibers are intended to impart rigidity to the composition, and are not intended to improve dynamic characteristics such as the tensile strength and elongation of the fiber-reinforced resin composition, and it is not intended to secure those dynamic characteristics as well as productivity and molding processability.
Japanese Patent Laid-open Publication No. 2004-91504 discloses a composition including a polyarylene sulfide, an aromatic polycarbodiimide-based resin and an inorganic filler. However, that composition is intended to improve moisture resistance and chemical resistance without considerably deteriorating dynamic characteristics, and while the dynamic characteristics of the composition are disclosed, a phenomenon in which the tensile strength and elongation of the composition is dramatically improved is not conceived. Further, it is not intended to secure those dynamic characteristics as well as productivity and molding processability.
Fiber-reinforced composite materials (FRPs) are light and excellent in dynamic characteristics, and are widely used in electric and electronic device applications, civil engineering and building applications, machine and mechanism component applications, robot applications, two-wheeled vehicle and automobile applications and aerospace applications. As reinforcing fibers to be used in those FRPs, metal fibers such as aluminum fibers and stainless fibers, organic fibers such as aramid fibers and PBO fibers, inorganic fibers such as silicon carbide fibers, and carbon fibers are used, and carbon fibers are suitably used because they are excellent particularly in specific strength and specific rigidity, and ensure outstanding lightness.
Typical forms of FRPs such as carbon fiber-reinforced composite materials (CFRPs) include molded articles produced by press-molding a prepreg, or a preform obtained by laminating prepregs (a molding method in which a prepreg or preform is defoamed under a pressurizing force, and shaped). The prepreg is generally produced by a method in which a reinforcing fiber base material with continuous reinforcing fibers arranged in one direction or processed into a woven fabric is impregnated with a resin, but discontinuous reinforcing fibers may also be used.
The molded article obtained by molding a prepreg produced using continuous reinforcing fibers has excellent dynamic characteristics, but it is not suitable to form a complicated shape because the reinforcing fibers are used in the form of a continuous body, and the use applications of the molded article are limited due to economic burden of the lamination step because its characteristics are greatly influenced by the lamination angle of the prepreg.
On the other hand, sheet molding compounds (SMCs) and glass mat base materials (GMT) produced using discontinuous reinforcing fibers are materials suitable for press molding, but their use applications are limited due to the problem that their dynamic characteristics such as specific strength and specific rigidity are low, adaptation to a thin molded article such as a prepreg is difficult, the rein considerably flows during molding so that isotropic dynamic characteristics are not achieved, and characteristics considerably vary, or the like.
Bondability between reinforcing fibers and a matrix resin influences the dynamic characteristics such as the tensile strength of the fiber-reinforced composite material, and therefore design of an interface between the reinforcing fibers and the matrix resin is very important in development of the fiber-reinforced composite material.
Japanese Patent Laid-open Publication No. 5-106163 proposes carbon fibers with a carbodiimide reagent deposited on the surface, and describes that the carbon fibers are excellent in bondability to a thermoplastic resin as a matrix resin, leading to improvement of the bending strength, but there is a problem in terms of interfacial strength between the reinforcing fibers and the matrix resin, and it is desired to further improve the strength.
Further, Japanese Patent Laid-open Publication No. 5-311069 discloses a technique relating to a surface modifier for carbon fibers including an organic compound having two or more carbodiimide bonds in the molecule, and carbon fibers modified with the modifier, but similarly, there is a problem in terms of interfacial strength between the reinforcing fibers and the matrix resin, and it is desired to further improve the strength of the molded article.
Further, Japanese Patent Laid-open Publication No. 2005-239917 discloses a resin composition including an aliphatic polycarbodiimide-based resin as a polyarylene sulfide resin and fibers as an inorganic filler, but the tensile strength of the molded article is insufficient, and it is desired to further improve the strength.
It could therefore be helpful to provide a fiber-reinforced resin composition having dynamic characteristics such as the tensile strength and elongation as well as productivity and molding processability, to provide a fiber-reinforced composite material excellent in dynamic characteristics such as the tensile strength.